1. Field of the Invention
The present invention relates to a flash memory with a power supply controller and a second controller for other control purposes as well as to a microcomputer incorporating such a flash memory, the power supply controller controlling voltages applied to the memory for data write, erase or verify operations thereon, wherein the power supply controller and at least part of the circuits in the second controller operate from the same supply voltage.
2. Description of the Prior Art
FIG. 6 schematically illustrates a conventional flash memory that operates from a single power supply. In FIG. 6, reference numeral 61 stands for a flash memory; 62 for a memory unit having memory devices arranged therein; 63 for a controller that controls the memory unit 62 in operation; 64 for a power supply controller that controls voltages applied to the memory unit 62 for data write, erase or verify operations thereon; 66 for a power supply (marked VCC hereunder) that supplies power to the memory unit 62, controller 63, power supply controller 64 and a port 73; 68 for a grounded circuit (marked VSS hereunder) covering the memory unit 62, controller 63, power supply controller 64 and port 73; and 73 for the port for exchanging data with an external circuit.
In the conventional flash memory 61, the VCC 66 acts as a common power supply that powers the memory unit 62, controller 63, power supply controller 64 and port 73. The VCC 66 is used by a step-up/step-down circuit in the power supply controller 64 to generate voltages for data write, erase or verify operations on the flash memory 61. The VSS 68 works as a common grounded circuit shared by the memory unit 62, controller 63, power supply controller 64 and port 73.
In the conventional flash memory controlled voltages generated by the power supply controller for data write, erase or verify operations on the memory unit are liable to be affected by operations of other control circuits in the controller. This makes it difficult to supply voltages and often leaves the ground potential unstable. Possible negative consequences include: a reduced write voltage prolonging the time it takes to write data to the flash memory; a lowered erase voltage prolonging the time required to erase data from the flash memory; a raised write or erase voltage destroying the flash memory; and a fluctuating verify voltage nullifying the verification of whether data is written to or erased from the flash memory correctly.
Microcomputers incorporating the above kind of flash memory are vulnerable to the memory-related irregularities listed above. In such microcomputers, the CPU may take more time to write and erase data to and from the flash memory or may otherwise develop memory-related malfunction.